FIG. 1 is a schematic diagram showing a torque detector disclosed in Japanese Patent Laid-Open No. 61-178627 for example, in which figure, (1) is a driven shaft to which a torque is applied, (3) are coils for detecting the amount of change in magnetic permeability, (4) are magnetic elements of which magnetic permeability change in accordance with the amount of the internal strain generated by the torque applied to the driven shaft (1). A plurality of magnetic elements in the form of strip pieces cut from a thin sheet of magnetic material are arranged in symmetry around the driven shaft (1) at an angle of .+-.45.degree. for example.
The operation will now be described When a Lorque is applied from the exterior to the driven shaft (1), a main stress having a main axis along the longitudinal direction of a group of the rectangular magnetic elements is generated. When it is assumed that this stress is a tensile force in the magnetic element group on the left side of the driven shaft (1) as viewed in FIG. 1, then the stress on the right side element group is a compression force. In general, when a stress is applied to a magnetic material of which constant of magneto-restriction is not zero, its magnetic property varies and accordingly the magnetic permeability changes as well known. This phenomenon is utilized in a magnetoristriction transducer for converting mechanical energy into electrical energy, and corresponds to Villari effect according to which the magnetic permeability of a magnetic material varies in accordance with the amount of deforming of the material. Also, when the magnetoristriction constant, which is an amount quantitably indicating the amount of magnetoristriction, is positive, the magnetic permeability increases when a tensile force is applied, and the magnetic permeability decreases when a compressive force is applied. It is also known that a quite opposite result is obtained when the constant of magnetoristriction is negative. Therefore, since the magnetic permeability of the magnetic elements (4) changes as they deform in accordance with the amount of the externally applied torque, the amount of the torque applied to the driven shaft (1) can be determined by detecting the change in magnetic permeability as the change in magnetic impedance by the sensor coils (3) wound around the driven shaft (1).
As a method for securely attaching the magnetic elements (4) on the driven shaft (1) of the torque detector of the above-described construction, a method in which the magnetic elements are formed from a thin sheet of a magnetic material and the magnetic elements (4) are bonded one by one to the driven shaft (1) at a predetermined angle thereto, or a method in which the magnetic elements (4) are bonded to a nonmagnetic film by a bonding agent or the like and then this film is wound around the driven shaft (1) and secured thereto by an epoxy thermo-setting bonding agent or the like.
FIG. 2 is a flow chart showing where the magnetic elements are bonded by a bonding agent onto a conventional non magnetic film. As seen from the figure, a strip of thin magnetic sheet material is cut into a plurality of magnetic elements (Step 41). Then, a bonding agent is applied on the plastic film (Step 42). The magnetic elements are placed on the bonding agent applied to the plastic film (Step 43), and the plastic film is wound around the driven shaft (1) (Step 44). The plastic film is secured by caulking at its periphery and thermally set to firmly attach the magnetic elements (4) around the driven shaft (1) (Step 45).
According to the conventional manufacturing method, a careful attention must be paid to arrange the magnetic elements at a predetermined angle with respect to the driven shaft (1). Particularly, when the plastic film is bonded by a thermo-setting bonding agent after the magnetic elements are bonded to the plastic film, it is necessary that the plastic film be of a heat-resistive material such as polyimide, Teflon, polyester/polyimide and that the bonded portion be firmly secured, allowing no movement. Particularly, when an amorphous magnetic material is used as the magnetic sheet material, a large external force is necessary to firmly and intimately secure them around the driven shaft (1) since the amorphous magnetic material is very hard and elastic. Also, the efficiency of the transmission of the stress due to the torque between the driven shaft (1) and the magnetic elements (4), that is, the magnitude of the stress propagation in the radial direction within the bonding agent layer depends upon the thickness of the bonding agent layer, so that it is desirable that the thickness of the bonding agent layer is thin and uniform in order to increase the sensitivity to the torque. In particular, when the magnetic elements are arranged in pairs in differential-type as shown in FIG. 1, the imbalance of the thickness of the bonding agent layer on the left and right directly affects the balance of the output, so that it is necessary to pay attention to ensure that the external force applied for securing as well as the amount of the applied bonding agent is equal between both left and right sides and is uniform throughout.
Particularly, when an imbalance appears in the thickness of the layer of the bonding agent on the left and right sides, this becomes an offset in the static characteristics of the sensor, which offset is difficult to be temperature-compensated because the offset amount increases and decreases according to temperature and is not an amount that can be estimated at the time of manufacture.
Since the conventional method for manufacturing a stress detector is as above described, the process conditions at the time of bonding must be sufficiently suppressed and the attaching conditions for the magnetic elements (4) must be made uniform throughout and symmetry in the left-and-right wise, so that a very reliable, delicate and complex technique is needed. Even with such technique, it is difficult to obtain a quality stress detector, posing problems of time-consuming adjustment, high process costs, etc.
This invention has been made in order to solve the above problems and has as its object the provision of a method for manufacturing a highly reliable stress detector in which the process for securing the magnetic elements (4) can be simplified, and the securing conditions can be uniform throughout and symmetric in transverse direction, and the dispersion of the detector quality can be decreased.